Devices for transmission of one-way torque are well known, common examples include a socket wrench and the winding mechanism of a clockwork motor, watch, clock etc. Generally such devices comprise at least one metal part and a biasing spring to bias a pawl in engagement with the teeth of a ratchet wheel.
WO90/13328 discloses a dry powder inhalation device comprising a housing defining a chamber in communication with a patient port in the form of a mouthpiece or nasal adaptor, and an elongate carrier bearing a powdered medicament, the device being constructed and arranged such that areas of predetermined size of the elongate carrier may sequentially be exposed within the chamber, the device comprising one or more air inlets such that when a patient inhales through the patient port an air flow is established from the air inlet(s) to the patient port through the chamber such that particles of the powdered medicament of respirable size from said exposed area of the elongate carrier are entrained within the air flow.
The elongate carrier is preferably in the form of a tape having a surface with grooves, pores, apertures or other embossed features which contain particles of medicament. The tape is conveniently wound on a supply spool and preferably contained within a cassette having a supply and take-up spool. The tape may contain many doses of the drug e.g. 200 doses.
In use, areas of the tape are sequentially advanced into the chamber to dispense the medicament contained within that area of tape. It is essential that the tape is advanced in well defined steps from the supply to the take-up spool to facilitate accurate, reproducible dosing and to prevent drug wastage. The advancement of the tape is conveniently facilitated by pivotal movement of a lever, either in a similar manner to the winder lever of a camera, or more preferably in the form of a mouthpiece cover, such that the tape is automatically advanced when the patient opens the cover. Such arrangements require an efficient one-way drive providing precisely controlled advancement and so that closure of the cover causes no movement of the tape.
The requirements of such a drive mechanism for use in an inhaler are manifold. It is desirable that the mechanism be cheap, preferably injection moulded in a minimum number of plastic parts. The device should be compact and lightweight, able to transmit large drive forces in relation to the torque needed to reverse the mechanism, capable of achieving low levels of variation in any lost motion, reliable, able to withstand temperatures of -20.degree. C. to +70.degree. C. for several hours without creeping or stress relaxing when "parked" in any configuration, able to resist wear and tear after prolonged usage (several thousand operations in each direction), reasonably immune to dirt or powder ingress, cheap to assemble, and quiet in operation.
The invention has been made with the above points in mind.